Conventional x-ray imaging techniques produce a shadow view of a body under examination. However, such a view does not contain sufficient information to specify the depth of a given point of interest in the body. Further, it is often impossible to distinguish small objects because they are obscured by the structure of larger overlying radio-opaque, objects, e.g., bones. These drawbacks have led to the development of body-section radiography systems known as x-ray tomography.
Conventional radiographic transverse tomography seeks to view planar sections or slices which are perpendicular to the axis of a patient. In such systems, a radiation source and associated detector means are rotated in a plane about a body under examination for measuring the absorption of the body at each of a plurality of angular positions. Using one of a number of well-known reconstruction algorithms on the data obtained by scanning the patient, a two-dimensional distribution of absorption coefficients is calculated for a slice defined by the plane within which the source and detectors are rotated.
The above-described x-ray imaging techniques require transmission of ionizing radiation through the body under examination. In order to minimize radiation dose to a patient, alternative imaging systems which reduce radiation exposure are employed in certain applications. Single-photon emission computerized tomography (ECT) is such an alternative imaging system. In carrying out an ECT session, the patient is first injected with a substance which, in itself, is innocuous, such as one of the natural substances of the body, and which is labeled with a radioactive tracer, or radioisotope. This tracer preferably has a short half-life so that very little radiation remains in the body after the examination has been completed. The tracer preferably emits gamma rays which are not readily absorbed by the body so that an appreciable part of the radiation will escape the body rather than be absorbed by it. The emission is preferably at a single energy so that detection can be made more easily and so that more accurate knowledge about the absorption factor of the body tissue through which the gamma ray has passed can be obtained.
Next, a gamma camera is used to scan the patient by moving the gamma camera around the patient in a closed, usually circular trajectory. The circular trajectory, e.g., enables the camera to obtain a series of measurements where each elemental area of the detecting surface of the camera traces a circular trajectory while the detecting surface is maintained perpendicular to the radius at all times, the radius being defined as the radius vector from the axis of rotation to the center of the camera. The simplest such rotation is of course one in which the axis of rotation coincides with the chosen axis of the coordinate system.
Emission data thus obtained while the camera is located at a particular predetermined angular position with respect to the axis of rotation, constitute a two-dimensional image that represents a view of the patient at the predetermined angular position. Such data are stored in what is termed herein, a memory frame. If a memory frame is obtained for each 3.degree. of rotation of the gamma camera, 120 such frames will be obtained by a complete circuit of the patient by the camera.
Related lines in each memory frame so obtained constitute data associated with a single plane perpendicular to the axis of rotation of the camera about the patient. The data in such lines thus are analogous to views that would be obtained were transmission scanning of the patient carried out in such plane. By processing the data contained in the memory frames using a reconstruction algorithm similar to that used in transmission tomography, one obtains images of transverse sections through the patient at many different axial locations along the axis of rotation. Because of the nature of the ECT technique itself and also assumptions made with regard to the reconstruction algorithm, the cross-sectional images obtained are of relatively poor quality. Consequently, while using the ECT technique to obtain data reduces the radiation dose to a patient as compared to using transmission tomography, reconstructed cross-sectional images produced from the data so-obtained do not permit an interpreter of such images to precisely locate a point of interest in the patient, and do not enable the interpreter to estimate the density of the radioactive matter at that point quantitatively.
It is therefore an object of the present invention to improve the precision with which a point of interest in a patient can be located employing data obtained using the above-described ECT or similar techniques.